Why do I have to use Handcent as my default to use the Private inbox??

[AC Question]

I downloaded Handcent for the Private inbox option. After I sent the contacts I wanted to be in the private box they still pop up in my phone's SMS inbox as well. Whats the point if I have to make Handcent my default app when I just want the privacy option? The thing is I want to use my phone's SMS system for regular texting & Handcent for my private texts WITHOUT having to make Handcent as a default?? Is this even possible?? Help!! I have a Galaxy S5 btw